Olefin monomers may be oligomerized to form oligomers. For present purposes, oligomer is defined as a series of at least two monomer units and at most roughly 300 monomer units (roughly 600 carbon atoms), and which is a liquid or waxy material at room temperature. Ethylene can be trimerized to form 1-hexene. Processes for oligomerizing olefin monomers typically employ catalyst systems comprising one or more catalytic metal compounds, typically transition metals, perhaps together with a co-catalyst and/or a support such as alumina or silica. Oligomerization processes are generally homogeneous processes. Processes for making 1-hexene from ethylene are typically homogeneous processes, in that the reactants and products are soluble in a diluent, such as isobutane.
It is desired to monitor and control the oligomerization reaction so that one may operate the process efficiently. Control of the concentration of monomer, and if present, comonomer and hydrogen, is required to ensure an efficient, reliable production process. Therefore, it is desirable to monitor the oligomerization process by determining monomer content and, when one or more co-monomers are present, by determining co-monomer content(s). It may also be desirable to determine diluent content and product content.
Current methods of monitoring oligomerization processes and the components in such processes (reactants, products and diluent) are less than optimal for several reasons. In processes for producing 1-hexene from ethylene, the contents of ethylene monomer, 1-hexene product, and by-products such as octene and decene, have typically been determined by gas chromatographic analysis of the flash gas, that is, the gas released at one of the flash tanks where pressure is released.
However, monitoring of an oligomerization process by analysis of the flash gas is less than optimal for several reasons. One reason is the amount of time for such analysis. If an analysis takes too much time, it generally has less value for monitoring, controlling or adjusting the olefin oligomerization process. Also, another concern arises when the oligomerization equipment includes more than one flash chamber, such as a high pressure flash chamber and a low pressure flash chamber. In such instances, gas chromatographic analysis of flash gas takes more time and is potentially less accurate when both high pressure and low pressure flash tanks are in operation.
While the contents of the oligomerization reactor may be determined by removing a small sample for analytical testing in a remote laboratory, this is less favorable than monitoring in situ. It may be dangerous and difficult to remove a sample from a hot process stream, and there are risks that the sample may not be representative of the overall reactor contents or that removing the sample may alter the sample. Sampling is time-consuming, and delay may cause the sample not to be representative of the reactor contents. A significant amount of material may be produced in the time required to remove, prepare, and analyze a sample. The analytical data obtained from the delayed sample is therefore of limited value for proactive process control. Furthermore, additional processing of the extracted sample may be required yet is undesirable.
A preferred method of monitoring a process for oligomerization, such as for making 1-hexene from ethylene, would monitor the process as it happens, or as soon thereafter as practical. It is also preferable that an analysis method be performed in situ, as opposed to being performed on samples removed from the oligomerization equipment. An in situ method would reduce the need to remove samples from the production environment, improve safety, and yield faster measurements. However, there are obstacles to providing in situ on-line chemical information in a process environment. The analytical method must be sufficiently accurate and precise under hostile physical and chemical conditions. The analytical method must be capable of remote detection and analysis.
Analyses of oligomerization processes in situ, that is, within a reactor or associated equipment, have been difficult if not impossible to do, since such processes are carried out at high pressures. However, spectrophotometric apparatus such as a spectrograph and a radiation source can be situated in a location remote from the reactor that is to be analyzed in situ, the sampling site being connected to the apparatus by radiation conduits comprising fiber optic cables.
Raman spectrometry can provide qualitative and quantitative information about the composition and/or molecular structure of chemical components. Raman spectrometry is based upon the vibrational energy of a compound. A sample is irradiated, preferably by a monochromatic light source, and the scattered radiation is examined through a spectrometer using photoelectric detection. Most of the scattered radiation has the wavelength of the source radiation, which is referred to as Rayleigh scattering. However, the scattered radiation also comprises radiation at shifted wavelengths, which is referred to as Raman scattering, which occur at different wavelengths due to molecular vibrations. The difference in wavelengths between the source radiation and radiation affected by molecular vibrations is commonly referred to as the Raman shift. Even if monochromatic light is used as the source radiation, the Raman spectrum will comprise scattered light spread across a wavelength band. The Raman shift or Raman spectrum conveys compositional and molecular information regarding the component in the sample. The Raman spectrum is extremely weak compared to the Rayleigh spectrum.
Not all substances are measurable by Raman spectrometry. There must be a change in polarizability during molecular vibration of a substance in order for that substance to be Raman active.
There are several factors that have favored the use of gas chromatography as an analytical method over Raman spectrometry with oligomerization processes. In general, the reactants and products in oligomerization processes may have peaks in their respective Raman spectra that are relatively close together. For example, in the trimerization of ethylene to produce hexene, the reactant ethylene and the product hexene will each produce similar peaks in their Raman spectra. Ethylene exhibits a peak at 1620 cm−1 while hexene exhibits a peak at 1640 cm−1. As a result, it may be difficult to distinguish between ethylene and hexene, and there is likely to be some overlap in certain peaks. Thus, it would appear necessary to employ high resolution Raman spectrometry equipment to analyze the components of the hexene preparation process. Furthermore, Raman spectrometry equipment, particularly high resolution Raman spectrometry equipment, is relatively expensive, which would generally discourage its use with industrial processes. Gas chromatography equipment has historically been less costly than high resolution Raman spectrometry equipment. Furthermore, gas chromatography sampling systems are well established. Also, gas chromatography equipment tends to provide information that is more readily usable, whereas Raman spectrometry equipment tends to produce information that requires additional analysis. Engineers and operators tend to prefer equipment that provides a relatively simple reading rather than a spectrum.
International Application No. PCT/AU86/00076, which is incorporated herein by reference, discloses monitoring the presence or concentration of one or more chemical components by using Raman scattering. Optical fibers are used to direct electromagnetic radiation to and from the monitored environment, so that the Raman detector may be remote from the monitored environment. It is stated that the Raman monitoring method is applicable to gases, liquids and solids, though no particular chemical components are disclosed as being monitored. It is also stated that it is necessary to examine the intensity of the scattered light at selected characteristic wavelengths. A band pass filter system is used, which has a series of narrow band pass interference filters each having a band pass between 100 cm−1 and 400 cm−1. Each filter is chosen to give maximum transmission of the Raman scattering of a particular component to be analyzed. This international application does not disclose the use of Raman spectrometry to monitor an olefin oligomerization process, or to measure olefin monomers. The international application does not disclose a method of monitoring the presence or concentration of more than one chemical component when those components have overlapping Raman spectra.
U.S. Pat. No. 5,652,653, which is incorporated herein by reference, discloses a method of on-line quantitative analysis of chemical compositions by Raman spectrometry. The method comprises simultaneously irradiating the monitored chemical composition and a reference material. The method applies predetermined calibration means to the standard Raman spectrum of the analyzed chemical composition to ascertain the chemical composition. The method is used for analyzing a polyester manufacturing process. A polyester manufacturing process generally has a liquid reaction mixture that does not include solids or a slurry. The patent discloses the construction of constitution-intensity correlation (CIC) multivariate calibration means. This is done by comparing a plurality of peaks at different wavelengths in the Raman spectra, which are preferably standard spectra, with a plurality of chemical compositions of known concentrations. The wavelengths selected for construction of a CIC depend on the spectral characteristics of the particular component whose concentration in a chemical composition is to be determined. For each component whose in situ concentration in the composition is desired to be monitored at any given time, a separate CIC calibration is prepared.
U.S. Pat. No. 4,620,284, which is incorporated herein by reference, relates to qualitative and quantitative analysis using Raman scattering for substances in gaseous, liquid and solid form to provide numbers, rather than spectra, denoting the amounts of the substances present. It is disclosed that reference spectra are used to establish a relationship between spectra region areas and concentrations of substances, and that composite reference spectra may be prepared. The patent discloses a hydrocarbon analyzer dedicated to “PNA” analysis as a particular embodiment, which determines the composition of a hydrocarbon in terms of three groups: paraffins, napthlanes, and aromatics. Among the prior art disclosed in that patent is work accomplished using the Raman effect to analyze hydrocarbons, including an article entitled “Determination of Total Olefins and Total Aromatics.” Similarly, an article entitled “Low-Resolution Raman Spectroscopy,” Spectroscopy 13(10) October 1998, discloses Raman spectrometric analysis of mixtures of organic liquids as well as petroleum products.
However, it is believed that Raman spectrometry has not been previously employed to monitor an olefin oligomerization process.